Methods for the stabilization of medicaments vary widely, depending on the type of medicament. The addition of a reducing agent to an easily oxidized medicament is a well-known example. Ascorbic acid has been stabilized against oxidation by the use of a mixed solvent having a decreased dissolved oxygen content. The degradation (by hydrolysis) of some anesthetic esters has been inhibited by the addition of caffeine, which was shown to complex with the esters. The series of "paraben" preservatives, e.g., methyl paraben, ethyl paraben, propyl paraben and butyl paraben, are well-known stabilizers used in numerous pharmaceutical compositions. U.S. Pat. No. 4,328,213, issued on May 4, 1982 to V. Ecker et al., for example, describes and claims their use in the stabilization of injectable labetalol formulations.
The antiemetic agents utilized in the stable formulations described and claimed herein are known compounds, having been described, for example, in published U.K. Patent Application 2,160,871 A, published Jan. 2, 1986, the disclosure of which is incorporated herein by reference.
Metoclopramide is a well-known antiemetic agent with a structure similar to the antiemetic agents utilized herein, except that it contains a methoxy group in the 2-position. The Physicians' Desk Reference, 36th edition, 1982, pages 1565-6, shows that the injectable form of metoclopramide sold at that time was stabilized with sodium metabisulfite.
Published U.K. Patent Application 2,158,714 A, published Nov. 20, 1985, confirms that injectable metoclopramide formulations were then stabilized with sodium metabisulfite. It goes on to point out that metoclopramide was being used in conjunction with cisplatinum chemotherapy for cancer, and that cisplatinum had been found to be incompatible with sodium metabisulfite. It states that it was found that, surprisingly, sodium metabisulfite could be eliminated from injectable metoclopramide formulations without unduly affecting their stability.
In Chem. Pharm. Bull., 8, 504 (1960), K. Ikeda reports the results of a study which showed increased stability of certain barbiturates in water-ethanol or water-methanol solutions having reduced dielectric constants, as compared with aqueous solutions of the barbiturates.
In Chem. Pharm. Bull., 8, (1960), K. Ikeda reports that the same barbiturates have increased stability in aqueous solutions of ethylene glycol, propylene glycol, glycerol, glucose, mannitol and sucrose. However, his studies showed that the stabilization could not be attributed only to the change of dielectric constant of the medium. Moreover, he refers to the work of E. S. Amis et al. in J. Am. Chem. Soc., 63, 2621 (1940) with the alkaline degradation of bromthymol blue. Amis et al. found that the reaction between negative bivalent dye ion and hydroxyl ion was in accordance with the theory in methanol-water and ethanol-water, but in glycerol-water mixture the activation energy was opposite to the theory on dielectric constant.
In J. Am. Pharm. Assoc., 48, 77 (1959), A.D. Marcus et al. refer to "the widespread use of mixed solvents in pharmaceuticals and the relative lack of information concerning the effects of such solvent systems upon the stability of the active ingredients". They point out that any assumption that replacement of part of the water by a non-aqueous solvent is some sort of a panacea is erroneous, and probably results from a lack of appreciation of the ability of non-aqueous solvents, especially those which are hydroxylic, to participate in, or otherwise influence, solvolytic reactions. In a study of the hydrogen ion catalyzed solvolysis of chloramphenicol in water-propylene glycol solutions, they found that the addition of propylene glycol to the aqueous solution increased the rate of solvolysis of the chloramphenicol.